A multi-color thermal transfer printing apparatus of the type to which the present invention generally appertains has an elongated thermal print head positioned close to and in parallel with a platen roller. During operation of such a printing apparatus, a recording medium such as typically a sheet of paper and a continuous length of ink film are held in contact with each other on the peripheral surface of the platen roller by means of the print head. The ink film has carried thereon a thin layer of thermally fusible image transfer materials or inks of different colors. The thermal print head has a multiplicity of heater elements which are selectively activated responsive to a set of image signals supplied from any image scanning or image data supply means. A multi-colored image represented by the signals thus supplied to the print head is formed on the recording medium by the pixels of differently colored inks transferred to the surface of the recording medium.
The image transfer materials or inks carried on the ink film are colored in, for example, yellow, magenta and cyan or yellow, magenta, cyan and black. These differently colored inks are applied in a succession of distinct areas arranged to recur along the length of the ink film and each coextensive with the recording medium on which a multi-colored image is to be reproduced. During printing operation for a single recording medium, the ink film is driven to endwise travel between supply and takeup reels so that adjacent three or four of the differently colored areas are successively brought into registry with the recording medium. Pixels of a single color which are to form part of the multi-colored image to be finally reproduced are thus formed on the recording medium by transfer of an ink from each of the three or four adjacent color areas of the ink film. The whole multi-colored image is in this manner formed on the single recording medium in three or four consecutive cycles of image transfer operation.
A typical example of a thermal transfer printing apparatus of the described type is disclosed in Japanese Provisional Patent Publication (Kokai) No. 60-32687. In a prior-art thermal transfer printing apparatus taught in this Publication, the platen roller is first turned to move a recording medium forwardly from a predetermined print-start position with respect to the platen roller and thereby form pixels of one color on the recording medium. On termination of the printing operation in the one color, the platen roller is driven to turn in the reverse direction to move the recording medium back to the initial print-start position for the formation of pixels of another color on the recording medium. During movement of the recording medium back to the initial print-start position, the thermal print head of the apparatus is spaced apart from the platen roller so as to allow the recording medium to move freely with respect to the print head.
For the formation of a multi-colored image on a single recording medium in this prior-art printing apparatus, the recording medium is thus first moved to the print-start position having its leading edge held in contact with the peripheral surface of the platen roller. The platen roller is then driven to turn in forward direction so that pixels of, for example, yellow are printed on the recording medium. The yellow-colored pixels are formed on the recording medium with the print head held in pressing contact with the platen roller across a yellow-colored area of the ink film moving with the recording medium on the peripheral surface of the platen roller.
After a yellow-colored component of the whole multi-colored image to be finally reproduced is thus formed on the recording medium, the print head is moved away from the platen roller and the platen roller is driven to turn in the reverse direction to move the recording medium back to the initial print-start position having its leading edge located between the platen roller and platen roller. The ink film is driven to endwise move a predetermined distance so that the magenta-colored area immediately subsequent to the yellow-colored area which has just been used is to be in registry with the recording medium. The print head is then moved into pressing contact with the platen roller across the recording medium and the magenta-colored area of the ink film.
The platen roller is then driven to turn in the forward direction for a second time so that magenta-colored pixels are printed on the recording medium. On the recording medium is thus formed a magenta-colored component of the whole multicolored image in addition to the yellow-colored component of the whole image. The print head is then moved away from the platen roller for a second time and the recording medium is moved back to the print-start position for a third time, while the ink film is driven to further move to a position in which the cyan-colored area subsequent to the magenta-colored area which has just been used is to be in registry with the recording medium. With the platen roller driven to turn in the forward direction, there is formed on the recording medium a cyan-colored component of the whole multi-colored image i addition to the yellow-colored and magenta-colored components of the whole image. A multi-colored image composed of yellow-, magenta- and cyan-colored pixels is in this fashion printed on a recording medium in three consecutive cycles of printing operation using three adjacent colored areas of an ink film.
In the prior-art thermal transfer printing apparatus of the described character, extra arrangement is made so that a trailing edge portion of the recording medium is pressed against the peripheral surface of the platen roller when the print head is spaced apart from the platen roller upon termination of the first or second cycle of printing operation. This arrangement is desirable for enabling the recording medium to reliably stay on the platen roller and to accurately move back to the initial print-start position with the print head spaced apart from the platen roller.
Such extra arrangement of the prior-art printing apparatus includes a pressing roller located to be engageable with a trailing end portion of a recording medium moved to the position with which a cycle of printing operation using a colored area of the ink film is to be terminated. A problem has however resulted from this arrangement in that the presence of the pressing roller on the trailing end portion of the recording medium reduces the coverage which the print head is allowed to have for printing each of the single colored components of a multi-colored image on the recording medium.
Another example of a multi-color thermal transfer printing apparatus of the type to which the present invention appertains is disclosed in U.S. Pat. No. 4,505,603. In a thermal transfer printing apparatus taught in this issued Patent, the thermal print head is arranged to be pivotally movable toward and away from an angular position operative to hold a recording medium and an ink film in contact with the peripheral surface of the platen roller. The platen roller in this prior-art thermal transfer printing apparatus is also driven to turn in forward direction to move a recording medium forwardly from a predetermined print-start position and form pixels of one color on the recording medium. After pixels of the one color are thus printed on the recording medium, the platen roller is driven to turn in the reverse direction to move the recording medium back to the initial print-start position for the printing of pixels of another color on the recording medium. The thermal print head of the apparatus is angularly spaced apart from the platen roller during backward movement of the recording medium and is pivotally moved into contact with the platen roller across the recording medium and ink film before the printing of the pixels of another color is started.
In the prior-art thermal transfer printing apparatus of this type, the recording medium and ink film are held together temporarily after the ink film is released from the thermal print head. This is useful for the stable and uniform transfer of ink from the ink film to the recording medium and is effected by means of a pinch roller located in conjunction with the path of the ink film leaving the platen roller. Past the pinch roller, the ink film which has been firmly attached to the recording medium moving forwardly from the platen roller is forced to move away from the recording medium and is thus separated from the recording medium.
The recording medium and ink film being thus held together for some time after the ink film has been released from the print head, pixels of any color can thus be transferred stably and uniformly from the ink film to the recording medium. This advantage of the prior-art printing apparatus is however impaired by the fact that the recording medium which has once been stripped from the ink film may be accidentally brought into contact with the ink film and stained with an ink objectionably transferred from the ink film while the recording medium is being moved back to the initial print-start position.
It may also be taken into account that the coefficient of friction of the image-carrying surface of a recording medium stepwise increases each time the recording medium encounters a cycle of printing operation and a single-colored component of a multi-colored image is printed thereon. If it happens that the recording medium being moved back to the initial printstart position is brought into contact with the ink film, the recording medium may be hindered from being moved smoothly toward the print-start position and may thus be caused to move irregularly with respect to the platen roller. This may result in deviation of the recording medium from its proper path of movement with respect to the platen roller and in failure in achieving exact registration between the individual single-colored components of the multi-colored image finally printed on the recording medium.
It may be further pointed out that the ink film allowed to remain on the recording medium after the ink film has been released from the print head is electrostatically attracted to the recording medium and may thus be caused to move with the recording medium after the ink film should have been separated from the recording medium. This would invite an occurrence of a jamming of the ink film and accordingly in a failure of the apparatus to operate properly.